Solvent extraction of c{11 {13 c{11 {11 aromatic hydrocarbons

ABSTRACT

In a solvent extraction system utilizing a counter-current extraction column and an extractive distillation stripper for recovery of C6-C8 aromatic hydrocarbons, the feed is divided into light and heavy fractions containing, respectively, C6-C7 and C8 aromatics. The light fraction is introduced to the extraction column at an intermediate level along with the overhead stream from the stripper column, while the heavy fraction is introduced to the extraction column at a lower level. This procedure avoids loss of C8 aromatic components to the raffinate.

' :1 I ranted States ate t 1 1 397649522 Pollock Got. 9, 1973 [54] SOLVENT EXTRACTION OF C C 3,634,537 1/1972 l-lutto 208/321 AROMA-TIC HYDRQCARBQNS 2,886,610 5/1959 Georgian 260/674 [75] Inventor: litlndre W. Pollock, West Chester, Primary Examiner Delbert E- Gamz Assistant Examiner-C. E. Spresser [73] Assignee: Sun Oil Company of Pennsylvania, Attorney-George L. Church et a1. f" Philadelphia, Pa.

22 Filed: Dec. 20, 1971 [57] ABSTRACT In a solvent extraction system utilizing a counter- [211 Appl current extraction column and an extractive distillation stripper for recovery of C -C aromatic hydro- [52] 11.8. CI 208/321, 208/323, 208/333, carbons, the feed is divided into light n heavy frac- 260/674 SE tions containing, respectively, C C and C [51] Int. Cl Cl0g 21/28, C07c 7/10 r m ic The light fra tion is introduced o the ex- [58] Field of Search 208/321, 323, 333; r tion lumn at an in rmediate level along with 260/674 SE the overhead stream from the stripper column, while the heavy fraction is introduced to the extraction col- [56] References Cited umn at a lower level. This procedure avoids loss of C UNITED STATES PATENTS aromatic components to the raffinate. 3,422,163 1/1969 Asselin 260/674 3 Claims, 3 Drawing Figures NAPHTHA FEED AROMATIC EXTRACT FRACTIONATOR EXTRACTOR STRIPPER 3 ,7 64,5 22 1 I 2 SOLVENT EXTRACTION or c.,--(:, AROMATIC HYDROCARBONS SUMMARY or THE INVENTION BACKGROUND OF THE INVENTION This invention relates to the processing of gasoline or naptha stocks containing aromatic hydrocarbons of the C C range with the aid of an aromatic-selective solvent. The invention constitutes a variation from processingprocedures heretofore proposed, whereby better recovery of the C aromatic constituents of the feed is achieved.

The selective extraction of aromatics from naphthas is practiced widely. The Udex process utilizing a glycol type solvent has wide commercial use for extracting aromatics of the C.;C, range. This process is described in Hydrocarbon Processing, Sept. 1970, p. 248. In conventional practice of this process the hydrocarbon feed stream containing all of the C C aromatics is introduced at an intermediate level into a countercurrent extraction column in which the solvent phase flows downwardly. The raffinate and extract phases are removed from the top and bottom of the column, respectively. The extract phases are sent to an extractive distillation stripper column from which saturated hydrocarbons together with some aromatic material is stripped overhead. This overhead material is returned to a lower part of the extraction column as reflux. A mixture of high aromaticity containing the C -C aromatics is removed at an intermediate level from the stripper column, while the solvent is recovered from the bottom and recycled to the top of the extraction column.

While the foregoing processing procedure results in high purity aromatic products and the recovery of substantially all of the benzene and toluene from the feed, there is a tendency for C aromatics to be lost to the raffinate. This is due to the fact that these are the aromatic feed components which are least soluble in the selective solvent. Also the stripper overhead used as reflux in the extraction column generally contains a substantial content of naphthenes, typically of the order of -40 percent by volume, which have a tendency to displace C aromatics from the solvent phase and cause these components to appear in the raffinate. As a result incomplete recovery of the C aromatics usually is experienced in conventional operation.

Variations have been proposed in the processing scheme wherein the feed is first fractionated into light and heavy fractions and these fractions are introduced at different levels into the extraction column or into separate columns operated in an integrated fashion. Examples of such proposals appear in the following US. Pat. Nos.: 3,037,062, C. G. Gerhold, issued May 29, 1962; 3,422,163, G. F. Asselin, issued Jan. 14, 1969; 3,468,792 and 3,468,793, I(.D. Uitti, issued Sept. 23, 1969.

In aforesaid US. Pat. No. 3,422,163 the feed material is divided into a light fraction and a heavy fraction, I

the light fraction is fed to the extraction column near the bottom along with overhead material obtained from the stripper column, and the heavy fraction is introduced to the extraction column at an intermediate The present invention provides a different way of operating an extraction system comprising a countercurrent extraction column and an extractive distillation stripper column so that loss of C aromatic components to the raffinate is avoided.

The procedure according to the invention involves dividing the feed material into a light naphtha fraction containing C C aromatic material and a heavy naphtha fraction containing C aromatics. The light fraction is introduced to the extraction column at an intermediate level, and the heavy fraction is introduced thereto at a lower level. In one embodiment the heavy fraction is introduced adjacent the bottom of the column to function as reflux; while in another embodiment it enters the column substantially above the bottom and a portion of the C C aromatic product is recycled and introduced adjacent the bottom to serve as reflux. The extract phase from the bottom of the extraction column is passed into the top of the stripper column wherein substantially all non-aromatic hydrocarbons are stripped out as overhead. This stripper overhead stream is recycled back to the extraction column, being introduced therein at the same or about the same level of introduction as the light naphtha fraction. The C -C aromatic product of high purity is withdrawn as a sidestream from the stripper column, while solvent is removed from the bottom and recycled back to the top of the extraction column.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flowsheet illustrating a conventional manner of recovering the aromatic components from naphthas, which figure is included for purpose of comparison.

FIGS. 2 and 3 are schematic flowsheets illustrating the two different embodiments of the invention referred to above.

DESCRIPTION The invention can be utilized for the processing of any naphtha or gasoline stocks containing aromatics of the C C or C -C boiling range, including straight run fractions, reformates, catalytic gasolines and the like. The process is particularly useful for recovering benzene, toluene and xylenes from stocks of relatively high aromaticity, such as catalytic reformates boiling typically in the range of 300F.

The conventional manner of extracting such stocks is illustrated in FIG. 1. Feed naphtha containing the C C and C aromatics is introduced through line 10 to countercurrent extractor 1 1 at an intermediate level and flows upwardly countercurrent to the aromaticselective solvent which is fed into the column near the top through line 12. The raffinate stream leaves the top of the column via line 13, while the extract phase passes from the bottom through line 14. The extract phase is fed into the upper part of an extractive distillation stripper 15 wherein non-aromatic components are stripped out leaving the column as overhead through line 16. This overhead product, which also contains a substantial proportion of the lighter aromatic components, is recycled through line 16 and introduced as reflux into the lower part of extractor 11. In stripper 15 the C C aromatic material is also removed, being obtained as a sidestream product through line 17. The

solvent recovered from the bottom of stripper 15 is returned through line 12 to the extraction column.

In processing catalytic reformate of 80295F. boiling range, for example, by the conventional procedure of FIG. 1, the overhead from stripper l6 typically will have a composition by volume as follows:

olefins 5-l5% aromatics (mostly C -C 30-60% naphthenes (mostly methylcyclopentane) 20-40% paraffins 10-20% When this material is returned to the extraction column as reflux through line 16, the naphthene components (mainly methylcyclopentane) have considerable tendency to displace the least soluble aromatic components from the solvent phase. These components are those of highest molecular weight, i.e. the C aromatics. A substantial loss of C aromatics to the raffinate phase which leaves the top of extractor 11 is therefore generally experienced.

FIG. 2 illustrates one embodiment of the present invention which provides a better recovery of the C aromatic components than is normally experienced in the conventional operation. This embodiment is particularly useful in processing catalytic reformates typically boiling in the range of 80-295F. The feed naphtha is first sent to a fractionator 20, as indicated by line 21, from which a light naphtha fraction containing aromatics of the C,,C range is taken overhead via line 22 and a heavy naphtha cut containing the C aromatics is removed as bottoms through line 23. Typically the light and heavy fractions separated from the reformate will boil in the ranges of 80-270F. and 270-295F., respectively, the heavy fraction will usually have a C aromatic content of 80 percent or more.

As indicated in FIG. 2 these light and heavy fractions are introduced into extractor 24 at different levels, with the light fraction entering near the middle and the C aromatic fraction being introduced through line 23 near the bottom as reflux for the countercurrent extraction operation. The aromatic-selective solvent enters near the top of column 24 through line 25 and flows downwardly countercurrent to the hydrocarbon phase. Raffinate is obtained from the top through line 19. Solvents useful for this type of extraction are well known and have been described in numerous patents; see, for example the patents cited hereinabove. Solvents of the glycol type, such as diethylene glycol, dipropylene glycol, and various polyethylene or polypropylene glycols generally are preferred, but any other known or suitable aromatic-selective solvents can be used.

The extract phase from extractor 24 is transferred from the bottom through line 26 to the top of stripper 27. The latter functions in a manner similar to the stripper of FIG. 1, except that substantially less material need be stripped overhead and this overhead material generally will contain a lower proportion of saturate hydrocarbons than in the conventional process. Again the C -C aromatic extract product is removed as a sidestream through line 28. The overhead stream, instead of being used as reflux as in the conventional process, is fed to the extraction column 24 at the same level or about the same level as where the light naphtha fraction enters. Thus, as shown in FIG. 2, the overhead stream is sent through line 29 to line 22 where it joins the light naphtha feed before entering the extractor.

This introduction of the stripper overhead recycle to extractor 24 at an intermediate level along with the light naphtha feed instead of near the bottom results in an increase in the yield of C aromatics in the aromatic extract. This beneficial result comes about because, in the FIG. 2 process, the naphthenic components of the stripper overhead do not have the opportunity to displace the higher aromatic components from the extract phase. Comparative material balance data given in the accompanying tables illustrate the benefit to be derived by operating in the manner of this invention.

FIG. 3 illustrates a modification of the process of FIG. 2. Most of the system of FIG. 3 is the same as that of FIG. 2 and the corresponding parts are identified by the same numerals. The difference resides in the material introduced into extractor 24 near the bottom as reflux. In the process of FIG. 3, a portion of the C -C aromatic extract from line 28 is recycled through line 30 to extractor 24 and enters as reflux near the bottom, while the heavy naphtha feed is introduced through line 23a substantially above the bottom but below the level of introduction of the light naphtha feed from line 22. This modification in procedure is particularly advantageous when the heavy naphtha fraction is not highly aromatic, as when straight run fractions are being processed. In such cases the use of the heavy naphtha fraction as reflux for the extraction, as per FIG. 2, tends to permit non-aromatic components to appear in the C -C aromatic product. Thus, while a high yield of the aromatics is obtained, the purity of the product may not be as high as desired. Use of the procedure of FIG. 3 allows both high yield and high to oto be secured.

Tables I, II and III give typical material balance data (based on 100 volumes of feed per hour) for the various streams of FIGS. 1, 2 and 3, respectively, when processing a catalytic reformate boiling in the range of 295F. obtained from a reforming process employing a conventional platinum-containing catalyst. For illustrating the embodiments of FIGS. 2 and 3 it is assumed that the reformate feed to the two processes are of slightly different compositions so that the heavy naphtha fraction in the FIG. 3 process has a somewhat higher content of saturated hydrocarbons (about 15 percent) than in the FIG. 2 process (about 10 percent).

TABLE I Matt-rial Balance for Reformutc Processing per Fig. 1

(Basis; I00 vols. feed/hr.)

Comparison of the data in Table I with that of Tables II and III shows that in either embodiment of the invention there is less loss of C aromatics to the raffinate and a correspondingly higher recovery thereof in the C -C aromatic product. It also shows that a substantial reduction is effected in the amount of material that needs to be taken overhead from the stripper.

TABLE ll Material Balance for Reformate Processing per Fig. 2

(Basis: 100 vols. feed/hr.)

level and flowing same upwardly countercurrent to the solvent;

c. feeding a heavy naphtha fraction containing principally C aromatic hydrocarbon to the column at a iiCtl Light Heavy Aromatic Stripper naphtha naphtha Raffinate product recycle level below the level of 1ntroduct1on in step (b) and Component (line 22) (line 23) (line 19) (line 28) (line 29) flow ng same upwardly; lei-ms L0 L0 d. withdrawing a raffinate stream from the upper part benzene f the C mn toluene 20.2 1.0 19.2 s o Ct aromatics 0.3 10.0 1.0 9.3 10 e. wlthdrawmg an extract phase stream from the bot- C: aromatics 1.0 0.3 0.7 naphthenes 29 l9 tom of sa1d column and lntroducmg same mm the rr oP 5 upper part of a str1pper column wherem substanother saturates 39.3 1.2 40.5

tially all non-aromatic hydrocarbons are stripped Total vol.lhr.= 87.8 12.2 64.3 35.7 50 out as overhead.

TABLE 111 Material Balance in Reforinate Fig. 3

(Basis: 100 vols. feed/hr.)

ti' N il l Light Heavy Aromatic Aromatic Stripper naphtha naphtha Ruffinale product recycle recycle ('omponent (line 22) (line 23:1) (line l9) withdrawn (line (line 29) olcfins I.() Ll) Ill benzene 6.5 (1.5 3.6 20 toluenc 20.2 1.0 19.2 l(l.8 5 n aromatics (L3 10.0 Lt) 9.3 5.2 (I, aromatics 0.3 0.7 0.4 ("r1 nuphthencs 2.) 2.9 l0 C;,(T..paralt'1ns l7.6 l7! 5 other saturates 39.3 2.0 40.5

Total vol.lhr. 117.0 13.0 04.3 35.7 20.0 50

While the embodiments of the invention have been f. recycling the stripper overhead stream to said exdescribed above for processing feeds of the C5-C traction column and introducing same therein at aromatic range, they are also applicable to the process-' about the level of introduction in step (b); ing of narrower boiling feeds of the C C aromatic g. recycling solvent from the bottom of said stripper range. In such case the feed is fractionated into a light Column for reuse as speclfled In p naphtha containing toluene and a heavy naptha on. and withdrawing from an intermediate level Of said taining C aromatics, and the fractions are processed in PP column a aromatlc produc't Stream the same manner as described. of high P y- The invention Claimed is: 40 2. Process accordmg to claim 1 wherein the level of introduction of said heavy naphtha fraction in step (b) 1 Process for extracting C -C aromatic hydro aris near the bottom of said extraction column, whereby bons from naphtha stocks containing the same which the heavy naphtha fraction serves as reflux. comprises: 3. Process according to claim 1 wherein the level of I f v a. passmg an aromattc-selecttve solvent downwardly fl fl y naphthifrzuon m S tep (b) through a countercurrent extraction column; 15 substantla ly above t e bottom 0 San extraction column and a portion of sa1d C -C aromatic product feedlng a llght naphtha fractlon Contammg p stream is recycled to said extraction column and introp y aromatlc hydrocarbon 0f the fraflge duced therein near the bottom to serve as reflux. and saturates 1nto sa1d column at an 1ntermed1ate it 

2. Process according to claim 1 wherein the level of introduction of said heavy naphtha fraction in step (b) is near the bottom of said extraction column, whereby the heavy naphtha fraction serves as reflux.
 3. Process according to claim 1 wherein the level of introduction of said heavy naphtha fraction in step (b) is substantially above the bottom of said extraction column and a portion of said C6-C8 aromatic product stream is recycled to said extraction column and introduced therein near the bottom to serve as reflux. 